Driveability rating method and system

ABSTRACT

The system carries out continuous in-vehicle data measurement and storage for subsequent processing. A given driving manoeuvre is sub-divided into facets and the measured vehicle data is processed to provide a local rating for each of the facets using empirically derived transfer functions. The local ratings are weighted and combined to provide an overall rating which is converted to a rating out of ten based on a further transfer function based on correlations between derived and subjectively observed ratings. As a result, driveability rating and engine calibration/development can be accelerated and targeted.

[0001] The invention relates to a driveability rating method and system.

[0002] Currently, driveability ratings for vehicles are determinedsubjectively by experienced drivers. A vehicle is driven through aseries of predetermined manoeuvres and, based on the performance of thevehicle, as determined by the driver, a rating between 1 (very bad) and10 (excellent) is provided for each manoeuvre. A non-exhaustive list ofmanoeuvres includes start time, start quality, pull-away (acceleratingfrom stationary), ‘tip-in’ or acceleration and ‘tip-out’ ordeceleration. Manoeuvres are assessed across more than one range—forexample tip-in may be examined for city traffic (less than 40 miles perhour/64 kilometres per hour), highway conditions (more than 40 miles perhour/64 kilometres per hour) or high revs per minute (more than 3000RPM). In addition vehicle performance can be rated for different ambientconditions, for example at very low or very high temperatures, or athigh altitude.

[0003] Driveability quality is a consequence of the engine or moregenerally speaking the powertrain characteristics which either do or donot allow the driver to enjoy driving his vehicle. Driveability can beadjusted by calibration of the powertrain (engine and gear boxelectronic control systems) but derives as well from intrinsic qualitiesof the hardware (driveline, engine supports, etc.)

[0004] Driveability should not be confused with drive and handlingcharacteristics, which derive from the body and suspensions qualities,or with NVH (Noise, Vibration and Harshness) which represents anotherfield of refinement, more linked to vehicle parameters. However, somelevels of vibration and noise can sometimes be improved by enginemanagement system calibration and are then mentioned during driveabilityassessments: they can concern for instance idle stability and noise ofdiesel engines in cold conditions, etc.

[0005] Because subjective means is typically used to rate driveability,it is likely to lead to the following problems: different points of viewpossible about what is acceptable and what is not; impossible toobjectively compare the assessments of different drivers since onlytheir ratings and comments are left after the tests; in the same way, itis difficult for one driver to compare the behaviour of severaldifferent vehicles, especially when there is a long time intervalbetween two assessments; difficulties in explaining and describing whata problem is or what the expected behaviour is, linked to wording andvocabulary difficulties, leading in the worst case to misunderstandings;and trouble for the calibrators to compare results at different stagesof the calibration and to objectively assess and quantify the progress.

[0006] For example, for many of the manoeuvres the experienced driver isattempting to sense various associated phenomenon which may affect therating provided for the manoeuvre. With start quality the driver looksfor conditions such as ‘overshoot’ in which the engine speed runs toohigh above the idle speed and then settles down to it or “undershoot”which can occur with the engine, upon return to idle, where by theengine speed drops below the idle speed and then returns to it. Inacceleration-type manoeuvres the driver may look for one of a range ofconditions including ‘thump’ for which there is an initial surge uponthrottle actuation, ‘shuffle’ in which there is an unacceptableacceleration oscillation during the manoeuvre or ‘hesitation’ where,once the throttle position is stabilised, there is a perceived hole inthe acceleration, that is, a drop in the rate of acceleration.

[0007] It will be apparent that the level of subjectivity ofdriveability rating can give rise to problems in consistency of ratingbetween drivers. Apart from simple driver variation, where a number offactors are taken into consideration in providing a rating for a givenmanoeuvre, the level of subjectivity may be further increased because ofthe individual driver's perception and assessment of the significance ofthe individual factors. For example, in a tip-in with high thump butminimal shuffle, a first driver may be more concerned with the latterfactor and provide a high rating whilst another driver may find thelevel of thump unacceptable and provide a low rating.

[0008] A known automated rating method is set out in U.S. Pat. No.6,079,258. The object of this arrangement is to develop algorithms fromreal driving assessments to allow driveability ratings to be derivedfrom vehicle measurements and to be applied on the test bed. In a firststep a driver takes a vehicle on the road and various parameters ofvehicle operation such as engine speed and longitudinal acceleration arecontinuously measured. In addition predefined ‘trigger conditions’ areset such that, off-line, appropriate parts of the recorded data can beevaluated defined by the trigger condition. In the evaluation stage aFast Fourier Transform is applied to the longitudinal acceleration datato identify the peak amplitude of surge oscillation in a frequency rangecorresponding to the transient operational phase of the engine. Thevalues thus derived are related to the driveability rating provided bythe driver using an empirically derived formula having one or moreconstants derived by an iterative process. The formula thus derived canthen be applied on the test bed. In this case the longitudinalacceleration is derived from the engine speed on the test bed using afurther self-learning process.

[0009] This arrangement suffers from various problems. It is dependentupon a number of approximations. Because of the manner in which theseapproximations are derived, there is no compensation for anomalouseffects which might occur outside the specific range for which thissystem is calibrated. Yet further the system relies on a highlysimplistic analysis of the vehicle measured values.

[0010] A further problem of existing driveability tools is that they donot give useful detail concerning how the rating was derived nor whatshould be improved practically to achieve satisfactory driveability infurther developments. As a result they produce results based on a blackbox system rather than providing an active tool to help thecalibrator/customer in their work; for example existing systems do notallow physical targets to be set up for a calibration project aftercomparing competitive vehicles in a benchmarking exercise.

[0011] According to the invention there is provided a method ofassigning a driveability rating to a predetermined manoeuvre in avehicle comprising the steps of:

[0012] identifying a plurality of manoeuvre facets;

[0013] recording vehicle performance data for the manoeuvre;

[0014] deriving a local rating for each manoeuvre facet from the vehicleperformance data; and

[0015] combining the local ratings to provide an overall driveabilityrating for the manoeuvre, in which the local rating is derived using apredetermined transfer function. As a result an accurate and efficientrating method is provided allowing a rating breakdown to be easilyprovided by accessing the local ratings. Yet further, the use oftransfer functions allows accurate and repeatable derivation of eachlocal rating.

[0016] Preferably the transfer function is derived empirically, as aresult of which the method will apply beyond the range of vehicleconditions for which the function was formulated.

[0017] A vehicle driver driving style factor may be derived from therecorded vehicle data and the local rating is derived taking the driverdriving style factor in account. As a result, the ratings are normalisedacross all drivers.

[0018] The gear ratio for the manoeuvre may be recorded, a related gearratio factor identified and the local rating derived taking the gearratio factor into account. Accordingly the manoeuvre does not have to berepeated in separate gears as this factor is normalised.

[0019] The local ratings may be weighted prior to their combination toform an overall driveability rating.

[0020] The combined local ratings may be subjected to an overalltransfer function to provide the overall driveability rating for themanoeuvre.

[0021] The local rating may be derived from a predefined valid portionof the vehicle performance data, such that only relevant parts of thedata require analysis. Preferably the predefined valid portion of thevehicle performance data is processed to provide representative dataprior to derivation of the local rating, and preferably the processingstep includes a linearisation step.

[0022] The invention further relates to a vehicle development methodcomprising the steps of assigning a driveability rating for a manoeuvreas set out above comparing the assigned rating with a target value and,if the target value is not met, identifying the respective local ratingsfor each facet of the manoeuvre, identifying those local ratings notmeeting respective target sub-values and identifying related vehicleperformance factors requiring development providing rapid and targetedvehicle calibration and development.

[0023] According to the invention there is further provided adriveability rating system for a vehicle in which, for a predeterminedmanoeuvre having a plurality of manoeuvre facets, vehicle performancedata for the manoeuvre is recorded in a data store or processor, a localrating is derived by a data processor for each facet from the vehicleperformance data; and the local ratings are combined in the processor toprovide an overall driveability rating for the manoeuvre in which thelocal ratings are derived using respective predetermined transferfunctions.

[0024] According to the invention there is yet further provided a methodof assigning a driveability rating to a predetermined manoeuvre in avehicle comprising the steps of:

[0025] deriving a rating function for a predetermined rating value;

[0026] identifying an error or proportional value between the derivedfunction and a measured function; and

[0027] deriving a rating based on the error or proportional value.

[0028] A simple and accurate rating system is therefore provided.Because the tool gives objective ratings for manoeuvres, based onphysical metrics extracted from the data, this enables it to overcomeproblems with the known systems. The main criteria subjectively used bythe drivers to assess driveability are summarised and given metrics bythe tool for each type of manoeuvre. This increases the driver's abilityto better understand what makes good or bad driveability. The tool canbe used during benchmarking exercises to objectively compare differentvehicle qualities, and to benchmark competitors vehicles. If such anexercise takes place at the beginning of a calibration project, the toolcan be used to set objective targets for driveability development. Forexample, the tool can be used to set values for the metrics ofmanoeuvres at levels that capture the customer targets for thecalibration work. The tool can then produce ratings from these values.During a calibration project, the ratings given by the tool reflect andquantify the progress toward the set targets. The tool can be used bydevelopers to quantify improvements in vehicle driveability; determinethe direction of the calibration effort; trade-off conflictingcalibration requirements, and demonstrate the achievement at key projectmilestones. Accordingly, the communication amongst the calibrators andbetween the calibration team and the customer is made easier.

[0029] Embodiments of the invention will now be described, by way ofexample, with reference to the drawings in which:

[0030]FIG. 1 is a flow diagram showing the manner of assigning thedriveability rating according to the present invention;

[0031]FIG. 2 shows pedal position (upper plot) and seat railacceleration (SRA—lower plot) against a common time axis for anacceleration delay facet;

[0032]FIG. 3 shows the accelerator pedal position (upper plot) and SRA(lower plot) against a common time axis for an acceleration average risevalue facet;

[0033]FIG. 4 shows pedal position (upper plot) and SRA (lower plot)against time for an acceleration decrease facet;

[0034]FIG. 5 shows accelerator pedal position (upper plot) and SRA(lower plot) against time for another example of the accelerationdecrease facet;

[0035]FIG. 6 shows pedal position (upper plot) and SRA (lower plot)against time for a total acceleration oscillation facet;

[0036]FIG. 7 shows a transport function for the acceleration delayfacet;

[0037]FIG. 8 shows a transport function for the acceleration rise facet;

[0038]FIG. 9 shows a transport function for the acceleration decreasefacet;

[0039]FIG. 10 shows a transport function for an acceleration oscillationfacet;

[0040]FIG. 11 shows acceleration pedal position (upper plot) and SRA(lower plot) for a vehicle manoeuvre for derivation of various localratings;

[0041]FIG. 12 is a plot of driver rating against tool rating accordingto the present invention;

[0042]FIG. 13 shows a transfer function for an ideal accelerationdecrease value against driver sportability s;

[0043]FIG. 14 is a transfer function for the acceleration decreasefacets;

[0044]FIG. 15 shows a transfer function for the acceleration maximumvariation facet;

[0045]FIG. 16 shows a transfer function for the acceleration totaloscillation facet;

[0046]FIG. 17 shows a plot of cranking time against engine coolanttemperature for a high cranking time rating R_(ref);

[0047]FIG. 18 shows the difference Δ between the high rating crankingtime and a lower rating cranking time against engine coolanttemperatures;

[0048]FIG. 19 is a plot of engine speed against time in engine start-up;

[0049]FIG. 20 shows plots on the same axes of engine speed versus timefor engine speed and differentiated engine speed;

[0050]FIG. 21 shows a transfer function for the engine speed riseregularity facet;

[0051]FIG. 22 shows a basis rating transfer function for engine speeddecrease regularity;

[0052]FIG. 23 shows a transfer function for a correction factor for theengine speed decrease regularity aspect;

[0053]FIG. 24 provides a transfer function for the engine start-up flarevalue aspect;

[0054]FIG. 25 is a plot of engine speed against time in engine start-upmode; and

[0055]FIG. 26 is a transfer function for the flare area aspect.

[0056] The method and system of the present invention is discussed inoverview with reference to FIG. 1 in relation to a typical situation inwhich a vehicle or engine in the development stages is being testedagainst a vehicle class benchmark as part of the development process. Insuch arrangements, the developers will want to assess whetherdriveability aspects are meeting the benchmark and if not, redesign orrecalibration may be required. Such redesign or recalibration may be inrelation to the engine hardware itself or the engine management system,but in either case the developers require detailed and targeted dataallowing them quickly to identify specific aspects of the engine orengine control that require modification and indeed even an indicationof the type of modification required.

[0057] At step 10, the driver takes the vehicle through a full range ofpredetermined manoeuvres and vehicle measurements are continuously takenduring driving.

[0058] These are stored in appropriate data storage means for subsequentoff-line data processing (although it will be recognised that this canbe on-line as well). The data processing takes place at step 12 in whichthe vehicle performance for various aspects or facets of the requiredmanoeuvres is assessed from the data.

[0059] At step 14, a first manoeuvre to be rated is identified and thevehicle performance data relating to each pre-defined facet of thatmanoeuvre is processed to obtain a raw numerical value for that facet atstep 16. Each numerical value is converted, using a transfer function toa local or intermediate rating value at step 18. At step 20 the localratings for each facet of the manoeuvre are weighted and combined toprovide a raw overall rating. At step 22 the raw overall rating isconverted to a standard rating out of 10 using a further transferfunction. This is then repeated for each manoeuvre.

[0060] The transfer functions used and aspects of measured vehicleparameters processed are empirically determined based on observations ofcorrelations between measured data and driver-perception of variousmanoeuvres and manoeuvre facets. As a result the system requires frontend design rather than significant on-line or off-line data processing.In addition, the system is reliable beyond the parameter range for whichthe transfer functions are compiled as it does not rely purely onidentifying mathematical relationships between measured values.

[0061] A detailed discussion will now be provided to explain how thetool is set up and used. The manoeuvres can be performed on anyappropriate manual/automatic vehicle.

[0062] In the first stage (to set up the tool), the driver takes thevehicle through the predetermined driveability manoeuvres and at thesame time a range of signals are recorded including: engine speed;vehicle speed; gear number (measured directly or deduced from engine invehicle speed); accelerator/throttle pedal position; clutch pedalposition; longitudinal acceleration; battery voltage; normal idle speed;engine coolant temperature. It will be recognised that additional valuescan be taken. The frequency and accuracy of data taken is dependent onthe resolution of the processing requirements subsequently and datastorage capacity. In this phase benchmark vehicles are used againstwhich a development vehicle is to be evaluated, and the driver rates thebenchmark vehicles for each manoeuvre performed. Significantly, in thisphase, not only the numerical driver rating is recorded but also thedriver's specific comments and criticisms. Based on these, as discussedbelow, transfer functions and ranges of data can be determined allowingan accurate correlation of measured vehicle data and manoeuvrerating/manoeuvre facet rating. This phase, during which the settings ofthe transfer functions are determined is preferably carried out using asmany drivers and as many benchmark vehicles as possible in order thatexperience of anomalous or additional parameter variations is gained toimprove the compilation of the transfer functions and the process as awhole.

[0063] The aim is to match each potential problem/quality with one orseveral features of some selected recorded signals, and to extractcharacteristic values from the signals which can be linked to the drivercomments. This is where the comments are more important than theratings. As one problem on a manoeuvre is enough to obtain a low mark,it is not always possible to match the quality of another specificaspect of the manoeuvre with the rating. For instance a tip in can beperfect in terms of shuffles but very slow. The rating will be poor andit will not reflect the absence of shuffles. If the driver's commentshave been systematically written down it is easier to check thepresence/absence of a given fault for the manoeuvre.

[0064] Once significant values have been extracted from the signals foreach defined feature, they must be compared with “ideal values”representing a perfect manoeuvre quality, which is achieved by using thetransfer function.

[0065] For each feature of the manoeuvre, a “local” rating is given,showing how close each aspect of the driveability of the car is to theideal behaviour. This local rating usually ranges between 0 and 1 (insome cases it can be negative in order to stress a particular badfeature): it is equal to 1 when the quality of the manoeuvre is perfectfor the specific feature, to 0 (or less) if the quality is very poor.

[0066] As the driver's rating is a subjective combination of severalresults that the car achieved for a given manoeuvre, in the same way thetool has to combine the local ratings to obtain an overall one. As somefaults are more badly rated than other ones, each local rating is givena weight, to calculate the global mark.

[0067] It will be appreciated that some aspects of the performance ofthe car will still be dependent upon individual driver characteristics.A clear example of this is one in which the driver manipulates theaccelerator pedal, as aggressive pedal manipulation will providedifferent readings to a more passive driving style. To filter out thisaspect of subjectivity an additional factor termed ‘sportivity’ isdefined, calculated from the data stored for the manoeuvres andincorporated into the subsequent processing steps to normalise thisaspect.

[0068] The second stage (in which the tool is used) is described in thefollowing with reference to the specific manoeuvres in a developmentvehicle: tip-in, tip-out and starts.

[0069] Tip-ins

[0070] All of the manoeuvres discussed can be defined by a limitednumber of criteria linked to subjective vehicle behaviour andexpressible in a physical way using metrics, automatically extractedfrom the recorded vehicle data by the system. Referring to FIGS. 2 to10, for example, tip-in quality can be fully assessed considering onlythe following facets: delay between the beginning of the pedal movementand the acceleration signal rise start, acceleration average rise value,acceleration decrease value once the pedal is stabilised and totaloscillations of the acceleration signal.

[0071] The first of these facets, the acceleration delay feature, partlycharacterises the responsiveness of the vehicle to a tip in. The idealvalue is zero delay between the beginning of the pedal movement and thebeginning of acceleration signal rise. This ideal value is independentof the driver's input. If the delay is too long the driver's commentscan be “hesitation” or “unresponsive”.

[0072] In this example the measured value of the delay between points 1and 2 in FIG. 2 represents the acceleration delay;

X _(point2) −X _(point1)=1161.71−1161.54=0.17s  (1)

[0073] The above calculation is the manual way of finding theacceleration delay. To integrate this calculation into a preferredsystem, it is necessary to determine the average levels of pedalposition and acceleration. Point 1 and Point 2 are then positioned whereboth signals have risen by a certain amount from their initial averagevalues.

[0074] As a result a numerical value is obtained for this specific facetof the manoeuvre, corresponding to step 16 shown in FIG. 1. This is nowconverted to an intermediate or local rating value using a transferfunction as shown in FIG. 7. The transfer function represents a localrating according to equation (2):

rating=max(1-delay, 0)  (2)

[0075] This function is a mathematical representation of providing aperfect rating of 1 corresponding to zero delay decreasing linearly to azero rating, representing very poor, if the delay is equal to or greaterthan one second.

[0076] The next facet of the tip-in manoeuvre is acceleration averagerise value. The expected behaviour of the car in this facet is closelylinked to the driver's input, that is to say the amplitude and therapidity of his pedal movement, which are contained in the “sportivity”which is derived and filtered out as discussed below.

[0077] The acceleration average rise value is determined between themoments when the accel signal starts to rise and when it stops rising,after the pedal is stabilised. At that stage even if the signal goes onslightly rising or decreases, its value will not be taken into accountfor the calculation. The average value is used and not an instant valuein order to get rid of potential very high slopes due to shuffles.

[0078] The slope of the accel signal during or immediately following thetip in is actually more physically perceived by the driver than theabsolute value of the acceleration once the pedal is stable. As for atip in it is more the transient management which is assessed rather thanthe engine torque value at the final pedal position; the way the enginereaches this final torque is important for the performance feel. Thismeans that the quicker the torque variation the better, which ismathematically expressed by the acceleration signal slope.

[0079] The acceleration signal is approximated with a series of straightlines. The slope of the line following the increase in the acceleratedpedal signal is taken on the average acceleration rise value. The nextstep selects the lines which have to be included in the calculation.

[0080]FIG. 3 shows the trace of accelerator pedal position in the upperplot and seat rail, i.e. linear acceleration in the lower plot, eachagainst a synchronised time value. The portion of the accelerationsignal relevant to the calculation is shown by region E and the slope ofthis is calculated to give an average acceleration rise value. The traceshown represents the simplest case in which linearisation provides asingle straight line portion in the relevant region. In more complexcases, linearisation may give more than one straight line portioneffectively representing a situation where the rate of accelerationchanges during the relevant portion. In such cases, in a preferredembodiment, the slope of the straight line between the first point ofthe first portion and the last point of the last portion is calculatedto provide the acceleration average rise value. It will be noted thatthe acceleration signal shown is noisy—this noise can be filtered outusing any appropriate noise filtering technique and lower frequencyvariations can further be filtered out using appropriate thresholdingtechniques

[0081] As discussed above, once significant values have been extractedfrom the signals for each defined feature for example accelerationaverage rise value, they must be compared with “ideal values”representing a perfect manoeuvre quality. For some criteria, this idealvalue is simple and constant. Taking the example discussed above of thedelay between the driver's movement of the accelerator pedal in a tip inand the vehicle's response, the ideal value for this metric is 0 secondsas represented in FIG. 7. For other criteria, the ideal value is morecomplex to define and a transfer function is required. These functionsare empirical and often include calibratable parameters depending on thespecific feature under consideration.

[0082] The input of these functions may depend on external conditionssuch as engine coolant temperature, or may depend on the level of thedriver's input. An instance of this is the slope of the accelerationsignal, one of the tip in criteria, which is a function of the driver'sinput. If the driver applies a small variation of accelerator pedalposition in a very smooth way, the expected response of the vehicle interms of acceleration rise will be much lower than if the driver applieda sharp and large increase in accelerator pedal position.

[0083] Therefore a variable called “sportivity”, a function of thedriver accelerator pedal input, is factored in. The higher thisvariable, the more sporty the driver. The value for the driver'ssportivity is calculated for each manoeuvre from existing vehicleperformance data if needed.

[0084] Sportivity is defined as a function of the amplitude of pedalmovement and its rate of change of position, i.e. acceleration.

s=Δ+acc  (3)

[0085] Where:

[0086] s=sportivity

[0087] Δ=total pedal position variation during the tip in (% of the wideopen throttle position)

[0088] acc=pedal acceleration $\begin{matrix}{{acc} = \frac{\Delta}{t^{2}}} & (4)\end{matrix}$

[0089] Where:

[0090] t=time between the beginning and the end of the pedal movement$\begin{matrix}{{{so}\quad s} = {{\Delta + \frac{\Delta}{t^{2}}} = {\Delta \times \left( {1 + \frac{1}{t^{2}}} \right)}}} & (5)\end{matrix}$

[0091] Where s is in the range 0 to 10,000 and has no physical unit.

[0092] Sportivity is one input of the function defining the “idealvalue”, i.e. the driver's expectations in terms of acceleration averagerise value. Another input is also the gear ratio: for instance thedriver does not expect the car to respond as quickly to a wide openthrottle in fifth gear as in first gear. In order to deal with only oneinput, sportivity, a single function is defined for all the gear ratios.This function gives the relation between ideal accel rise value dividedby gear ratio and sportivity.

[0093] This function is empirically defined as follows: $\begin{matrix}{F = \frac{ideal\_ value}{gear\_ ratio}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}\left( {6a} \right)} \\{= {{Max}\left( {{b - \frac{1}{\left( \frac{s}{10000} \right) + c}},a} \right)}} & {\left( {6b} \right)}\end{matrix}$

[0094] a, b and c are calibratable parameters which are derivedempirically to match functions (6 a) and 6(b). They allow a singlefunction independent of the gear used.

[0095] The gear ratios used are not the number of each gear butrepresent the proportionality between each ratio: $\begin{matrix}{{gear\_ ratio} = \frac{\frac{{engine\_ speed}\quad ({rpm}) \times 10}{{vehicle\_ speed}\quad \left( {{km}/h} \right)}}{\frac{{engine\_ speed}{\_ gear1} \times 10}{{vehicle\_ speed}{\_ gear1}}}} & (7)\end{matrix}$

[0096] As a consequence the ratio for gear 1 is unity and the remainingare derived for a given vehicle based on its known ratios.

[0097] After determining the measured average acceleration rise valueand the ideal one, they are compared to find to which extent the carfulfilled the expectations of the driver in terms of acceleration rise.

[0098] If the measured value is equal to or greater than the ideal one,this feature of the signal is assessed as perfect and rated with a 1.

[0099] If the measured value is lower than the ideal one, the ratingdecreases in proportion to the difference between the two values. Itreaches 0 when the difference is equal to or greater than a calibratableparameter. This is represented by Equation (8) and the transfer functionis shown at FIG. 8: $\begin{matrix}{{Rating} = {\max\left\lbrack {{\min\left( {{1 - \left( \frac{\frac{{ideal\_ value} - {measured\_ value}}{gear\_ ratio}}{d} \right)},1} \right)},0} \right\rbrack}} & (8)\end{matrix}$

[0100] where d is calibratable and derived empirically for a givenvehicle and may for example be 1.

[0101] The input of this function is the difference between the idealacceleration rise divided by the gear ratio and the measuredacceleration rise divided by the gear ratio. The fact that the gearratio appears in the input enables a single value “d” to be used for allthe tip ins, whichever gear the manoeuvre is performed in.

[0102] Having obtained a further intermediate rating value or localrating for the acceleration average rise value, the step is repeated forthe acceleration decrease value. Theoretically, this value should alwaysbe equal to zero. It represents the negative variations of theacceleration signal after its main rise, once the pedal position isstabilised.

[0103] This value is measured between the end of the rising part of theacceleration signal and any new input from the driver: accelerator orbrake pedal movement for instance. Sometimes the driver slightlyreleases the accelerator pedal just after the tip in because he realisesthat he moved the pedal too far. The starting point of the analysis isthen delayed until the pedal is stable and the consequences of itsmovement on the accel signal are over.

[0104] The acceleration signal is linearised in the same way as for theacceleration rise analysis discussed with reference to FIG. 3 and theslopes of each straight line portion are calculated. The value left isthe lowest one obtained in the analysis window. No average value iscalculated since the window is usually quite large (several seconds)whereas a very brief acceleration drop is enough to create a badimpression on the driver. His impression is affected by sharp and shortduration changes in acceleration and this is what the analysis attemptsto capture. An average value would “dilute” the result and would notexpress the driver's feeling.

[0105]FIG. 4 represents traces of accelerator pedal position (uppertrace) and longitudinal acceleration (lower trace) in one possiblescenario. In this case, the region for which the acceleration decreasevalue is measured is designated B. Its slope is simply measured, afterappropriate filtering, to derive an acceleration decrease value. In theembodiment shown in FIG. 5 a significant acceleration decrease is shownin the region designated C although it can be seen that the acceleratorpedal position at that time is constant. A higher value for theacceleration decrease value will therefore be measured.

[0106] Once the measured accel decrease value is determined, it ispossible to compare it with the ideal value, which is 0, and give arating for this particular feature of the signal. The function used tocalculate the rating is shown in FIG. 9 as discussed below.

[0107] If the acceleration decrease is equal to zero or negative (theacceleration signal actually rises), this is the right behaviour and asa consequence the rating is 1. Physically the drivers accept, or cannotfeel, a slight decrease of the acceleration, but as soon as they canperceive it, their rating falls sharply as shown in FIG. 9 as thisbehaviour is considered unacceptable. The shape of the rating functionreflects this fact, with an “acceptance band” after which the rating isclose to 0.

[0108] The transfer function is represented by the following equation:$\begin{matrix}{{Rating} = {\frac{1}{2} \times \left( {1 - {\tanh \left\lbrack {10 \times \left( {{\min \left( {{\max \left( {{measure},0} \right)},e} \right)} - f} \right)} \right\rbrack}} \right)}} & (9)\end{matrix}$

[0109] Again e, f are derived empirically in the calibration phase.

[0110] The final facet of the tip-in manoeuvre for which a local ratingis obtained is the total oscillation of the acceleration signal. This isan important feature of the acceleration signal which characterises avery common transient problem: thump and shuffles. When this problemoccurred during the tests, the driver's comments may be “thump”,“harsh”, “shunt”, “bump”, “oscillations”, “shuffly” or “surgy”.

[0111] These unwanted excursions of the acceleration signal usually takeplace during the “accel rise” part of the tip-in. In very baddriveability cases, they can persist during the “stable” part. They arequantified adding up all the vertical movements of the accel signal,provided that they are greater than a threshold.

[0112] As shown in FIG. 6 a number of vertical movements V1 to V6 areidentified again using appropriate thresholding techniques. These aresummed to give a value of overall distance moved (in one direction).

[0113] If the acceleration signal was perfectly smooth, the previouscalculation would lead to a value equal to the variation of meanacceleration before and after the tip in: “delta accel” on the figure.Therefore delta accel is the ideal value the total oscillationcalculation has to be compared with so as to evaluate the correspondingrating. The rating is 1 if the measured value of the accel signaloscillations is equal to delta accel. It decreases proportionally withthe difference between the two and reaches 0 when this difference isequal or greater than a calibratable value. This is represented by FIG.10 and Equation (10). $\begin{matrix}{{Rating} = {\max \left\lbrack {{1 - \frac{\max \left( {{{measure} - {delta\_ accel}},0} \right)}{e}},0} \right\rbrack}} & (10)\end{matrix}$

[0114] Where e is derived empirically in the calibration phase and canbe, for example, 2.

[0115] The measured “total oscillations” value includes in the samevariable thumps (i.e. initial surge) as well as oscillations (i.e.continuing variation) even though the calibrations to change to correctthose two problems can be different. But physically, in terms of thedriver's feelings, both phenomena are very similar.

[0116] A thump without shuffles would lead to a lower calculated valuethan a tip in provoking thump plus shuffles. As a result the ratingwould be higher in the first case than in the second one. In practisedrivers too rate tip ins more severely if they are shuffly than if theyinclude a thump only, as a result the empirically derived rating isrepresentative.

[0117] As a result it can be seen that, for the tip-in manoeuvre-orperhaps one version of a manoeuvre between a specific range of RPM or ata specific speed, a number of manoeuvre facets have been identified andintermediate rating values or local ratings have been calculated foreach of those facets. The calculations have been based on identifyingrepresentative portions of the data recorded for the vehicle during themanoeuvre, filtering or otherwise manipulating the data to remove noiseas well as known driver-induced anomalies and normalising in respect ofdriver specific perturbations such as ‘sportivity’. The values thusderived are compared against an ideal value and provide a local ratingbetween zero and one using an empirically derived transfer function. Insome cases the transfer function has a linear relationship but in othersalternative functions are derived to take into account non-lineareffects such as, for example, driver tolerance of acceleration decreaseafter stabilisation of the accelerator pedal.

[0118] Once all the individual ratings are calculated for a givenmanoeuvre, it is possible to obtain the overall rating for it. This isthe sum of all the individual weighted ratings. The same weights areapplied for all the manoeuvres of the same type, i.e. all the tip-inratings for instance are calculated using the same weights. Their sum isequal to the number of studied features for the type of manoeuvreassuming that the local rating has a value up to 1. These weights areempirical and result from the drivers' comments and ratings analysis: ifa poor aspect of a manoeuvre is considered less acceptable than anotherone, then the weight for this feature must be relatively high.

[0119] An example of a global rating calculation is now discussed withreference to the trace shown in FIG. 11. The relevant manoeuvre is a 50%tip-in, first gear, to 25 miles per hour (40 kilometres per hour). Table2 shows, across the top, the various facets of the manoeuvre for whichratings are derived and any delay, average acceleration rise, averagedeceleration decrease and total oscillations. Along the side are shownthe measurements of initial value, the ideal value, the local ratingderived between zero and one (‘the marks’) and the weighting attached toeach of the local ratings. TABLE 2 Total Average accel Average acceloscilla- Delay(s) rise (g/s) decrease (g/s) tions (g) Measurement 0.140.26 0.07 0.96 Ideal value 0 0.31 0 0.34 Mark (0 to 1) 0.86 0.91 1 0.62Weight 0.5 1 1.25 1.25

[0120] The average rating is obtained from equation 11.

Average rating=ΕMark×Weight  (11)

[0121]FIG. 12 shows a transfer function according to which the ratingout of 4 is converted to a rating out of 10. This transfer function isderived from correlating the automatically derived ratings andsubjectively derived drivers ratings over a number of equivalentmanoeuvres.

[0122] As a result it will be seen that the invention allows anaccurate, repeatable and useful driveability analysis and developmenttool. Once the vehicle has been taken through the various manoeuvres, ifaspects of its driveability are falling below the target values for agiven manoeuvre, then the individual manoeuvre facets can be examined toestablish which of those have contributed to the low rating. From thisthe developers can identify an appropriate development strategy forimproving that factor by modifying the vehicle hardware or the enginemanagement system. Once the transfer functions have been developedempirically and any calibratable values are identified for a givenvehicle/engine then the vehicle can be test driven and the variousratings obtained can be compared against target ratings as part of thedevelopment process.

[0123] Tip-outs

[0124] It will be appreciated that the approach discussed above can beapplied equally well to any type of driving manoeuvre for whichdriveability ratings might be required and this will be examined below.For example, in relation to tip-outs, these can be divided intomanoeuvre facets comprising average acceleration decrease, maximumacceleration variation and total acceleration oscillation.

[0125] The calculation of the average acceleration decrease valuecorresponds exactly to calculation of the average acceleration increasevalue discussed above with reference to FIG. 3. Once the slope of thedecreasing portion of the acceleration signal has been calculated asdiscussed above, however, the transfer function is as shown in FIG. 13.The acceleration decrease is strongly linked with safety feel.

[0126] However, the car's tendency to brake when the accelerator pedalis released is extremely variable from one make to another. This is onereason why calibratable parameters have been introduced in the functionsused by the tool to evaluate the ratings according to the customer'swishes. Concerning the value the driver expects, it is quite clear thatit depends on the gear engaged as well as on the driver's input. If theaccelerator pedal is released very slowly or only partially, or if fifthgear is engaged, the driver expects a smoother back out than if thepedal is released quickly and completely or if first gear is engaged.Because the gear influences the ideal value, in order to have a singlefunction for all the gears, all the variables are divided by the gearratio, as defined in Equation (7).

[0127] As the expected value depends on the driver's input, thesportivity variable “s” (Equation (6)) is the input of the functiondefining the ideal accel decrease value, as set out in Equation (12),and represented in FIG. 13: $\begin{matrix}{{ideal\_ value} = {{\left( {f - g} \right) \times \frac{s}{10000}} + g}} & (12)\end{matrix}$

[0128] where f and g are again calibratable parameters, representativevalues being f=3, g=1.4

[0129] The rating of this characteristic of the accel signal is simplyproportional to the ratio between the measured and the expected values,as shown in FIG. 14. So if the acceleration decrease is equal to orgreater than the ideal value, the rating is 1. If it is lower, therating is lower as well, as shown in Equation (13): $\begin{matrix}{{Rating} = {\min \left\lbrack {\frac{\frac{{measured\_ accel}{\_ decrease}}{gear\_ ratio}}{ideal\_ value},1} \right\rbrack}} & (13)\end{matrix}$

[0130] The next facet is acceleration maximum variation whichcontributes to the “back out” feel (i.e. the extent to which the carbrakes on deceleration) as much as the average acceleration decreasevalue does. The analysis of the data reveals that drivers expect arelatively high instantaneous variation of acceleration for asatisfactory tip out, whatever the amplitude or quickness of the pedalmovement. On the acceleration signal, this means the presence of anundershoot. This type of expectation is opposed to the ideal tip-inbehaviour, where an acceleration overshoot, that is to say a thump, isusually unwanted.

[0131] Accordingly, a value delta_max is derived from the accelerationtrace corresponding to the maximum drop in acceleration once the pedalis released.

[0132] The only input for the ideal value of acceleration maximumvariation is the gear ratio. As it is a constant parameter, it ispossible to derive directly the rating of this feature versus themeasured delta_max divided by the gear ratio as shown in FIG. 15 andEquation (14): $\begin{matrix}{{rating} = \frac{{\max \left\lbrack {{\min \left( {\frac{delta\_ max}{gear\_ ratio},i} \right)},h} \right\rbrack} - h}{\left( {i - h} \right)}} & (14)\end{matrix}$

[0133] Representative values for h and i are respectively 0.6 and 1.4.

[0134] The measurement of acceleration total oscillation during tip-outis equivalent for that in tip-in as discussed with reference to FIG. 6.In particular a total value of change in acceleration delta_accel iscalculated in the same manner. However, there are two importantdifferences in the way the rating is evaluated in the case of tip outs.

[0135] First, drivers are observed to be less indulgent with shuffles inhigh gears than in low gears. Gear ratio (Equation (7)) must beintroduced in the calculation of the input variable of the ratingfunction, in order to have a single function for all the gears.

[0136] Second, as a thump is expected at tip-outs, a “tolerancethreshold” is used so that the ideal value for oscillations is not incontradiction with the ideal value for accel max variations.

[0137]FIG. 16 shows a representative trace and equation (15) shows therelationship in use. $\begin{matrix}{{rating} = {\max\left\lbrack {{1 - \frac{\max \left( {{\frac{{total\_ oscillations} - {delta\_ accel}}{gear\_ ratio} - j},0} \right)}{\left( {k - j} \right)}},0} \right\rbrack}} & (15)\end{matrix}$

[0138] j and k are calibratable parameters.

[0139] Starts

[0140] One final example of the driveability rating approach of thepresent invention is given for a slightly different aspect, namelystarts, which has two aspects rated entirely separately, cranking timevalue and start quality.

[0141] The clear requirement for cranking time is that the shorter it isthe better, taking into account that the lower the engine codanttemperature the longer the expected cranking time will be.

[0142] The definition of the cranking time is the time between thebattery voltage drop and the moment when the engine speed rises above aninitial value, this value being calibratable.

[0143] A curve of good cranking time as a function of temperature isshown in FIG. 17 used on a reference high rating R_(ref).

[0144] a, b, c, d are calibratable parameters. A similar curve,developed for a lower rating may be developed.

[0145] The curve shown in FIG. 18 presented below shows the difference(A) between a measured cranking time worth a low rating R_(low) and theexpected higher one R_(ref). e, f, g and h are calibratable time values.

[0146] To obtain the rating for a start time at a given engine coolanttemperature, the tool interpolates the cranking time for R_(ref) at thattemperature from FIG. 17 to obtain CT_(high) for that temperature andthen Δ for that temperature from FIG. 18 as a normalising factor.

[0147] The final rating for the measured cranking time—CT_(measured)—iscalculated with Equation. (16): $\begin{matrix}{{Rating} = {\min\left\lbrack {{\max\left( {{R_{ref} - \frac{\left( {{CT}_{measured} - {CT}_{high}} \right) \times 2}{\Delta}},0} \right)},10} \right\rbrack}} & (16)\end{matrix}$

[0148] This formula gives a value between 0 and 10, directly comparableto the driver's rating.

[0149] Start quality is evaluated according to a more complex range offeatures, some of which can be seen in the trace of engine RPM againsttime shown in FIG. 19 identified as: engine speed rise regularity(region D); engine speed decrease regularity (region E); flare value;flare area (region F). Empirically it is observed that this is aparticularly subjective area in that different drivers clearly weightdifferent aspects differently, but it is found that measurements of thefour facets discussed above provides appropriate levels of objectiverating derivation.

[0150] Engine speed rise regularity concerns the shape of the enginespeed between the starter plateau and the “flare” as discussed below.The speed rise should normally be neat and sharp, without stumble ormisfire, in order to give confidence that the engine will not stall justafter cranking. Otherwise, the drivers' comments can be “poor run upafter start”, “stumble”, “hesitation on run up”, etc.

[0151] To calculate the local rating for this feature of engine start,the system filters the logged engine speed then differentiates it. Theoscillations of the differentiated engine speed are afterwardsevaluated, principally in the same way as with the seat railacceleration facet for tip ins and tip outs (see for example FIG. 6):however only oscillations above a threshold are taken into account, andbefore they are added up, they are divided by the value of thedifferentiated engine speed at that point.

[0152] This division normalises the ratings which depend on enginetemperature; when the engine is very cold, with temperatures far below0° C., the observed slopes of engine speed are lower than in hotconditions, so the top values of the differentiated engine speed arealso lower, as well as its oscillations, even if the run up is poor. Asa consequence, the differentiated engine speed variations have to bemade relative to the differentiated engine speed value before they areadded up, otherwise the result of the calculation can be too similarbetween a bad cold start and a very sharp hot start

[0153] Referring to FIG. 20, the curve C₁ is the engine speed and curveC₂ is the differentiated engine speed. The relative oscillations of C₂are calculated between the end of the starter plateau and the flare. Forinstance, between the spikes y_(i) and y_(i+1), provided thatabs(y_(i+1)−y_(i))>threshold (a calibratable value), the relativevariation of the differentiated engine speed will be evaluated usingEquation (17): $\begin{matrix}{{Delta\_ relative} = {{abs}\left( \frac{y_{i + 1} - y_{i}}{\max \left( {y_{i + 1},y_{i}} \right)} \right)}} & (17)\end{matrix}$

[0154] If Delta_relative is close to zero, it means that the oscillationis not too high (y_(i) and Y_(i+1) being measured from zero). If it isequal to one, it can mean, depending on the specific case, that forinstance the derived engine speed dropped from its original value downto zero. As a consequence, the engine speed stops rising for a while,which is perceived by the driver as a hesitation. If Delta_relative isgreater than one, it can mean that the differentiated engine speeddropped down to a negative value, in that case the engine speed not onlystops rising but decreases for a while, which is typical of a stumble.

[0155] Once each separate Delta_relative is calculated for all thevariations of the derived engine speed between D₁ and D₂, they are addedup. The higher the final value is, the worse the rating of engine speedrise regularity will be, as represented by FIG. 21, where i is acalibratable value, for example i=15.

[0156] Engine speed decrease regularity concerns the behaviour of enginespeed between the flare and the point of return to idle. Ideally, theengine should return smoothly to the nominal idle speed, withoutundershoot. The return to idle should not be in two or more steps orworse, oscillatory. In case of wrong behaviour, the driver's commentsmay be “bounces on the way down”, “2-stage return to idle”, “return insteps”, etc.

[0157] To quantify the speed decrease irregularities, the system firstcalculates the sum of the positive variations of the engine speed,between the flare and the end of the return to idle (see FIG. 20). Thisvalue should be equal to zero since the engine speed is not supposed torise again during the return to idle. Once this result is obtained, afirst basis mark is given according to FIG. 22. It will be seen thatthis mark can be negative to have more impact on the final rating incase of really undesirable behaviour. j is a calibratable value, forexample 100.

[0158] Then the cumulative variations of the differentiated engine speedbetween the flare and the end of the return to idle are added up, in thesame way as for the tip in oscillations as discussed with reference toFIG. 8, using a threshold equal to zero. The result is representative ofthe engine speed irregularity after the flare and is used to provide acorrection factor for the basis rating previously calculated. Thiscorrection factor is evaluated as shown in FIG. 23. k and l arecalibratable, for example k=300, l=3500.

[0159] The final rating of engine speed decrease regularity is obtainedby multiplying the basis rating and the correction factor together. Inthe case of a negative basis rating, the multiplication is not applied,otherwise it would improve the final rating, the correction factor beingincluded between 0 and 1. In that case the final rating is set to thebasis rating, according to the following relation:

If Basis rating>0, Rating=Basis rating X Correction factor ElseRating=Basis rating  (18)

[0160] It will be seen that this final rating can also be negative asthe basis rating can be.

[0161] The flare value represents the difference between the maximumengine speed after the rise and the nominal idle speed (see FIG. 25). Areasonable amount of flare will give confidence to the driver that theengine is starting happily and easily. Little or no flare will give theimpression that the start is weak, too much flare is noisy and soundslike an uncontrolled engine. The measurement of the flare value isstraightforward from the analysis of the engine speed, as well as therating of this start characteristic, referring to FIG. 24. m, n, o, pand q are calibrated and respective representative values are 0.75,−200, 200, 500 and 1000.

[0162] Flare area is very much linked to the flare value, and, in alower proportion, to the engine speed decrease regularity. Itcharacterises how long the engine speed hangs up before returning toidle. If this time is too long, it gives the feeling of an uncontrolledengine.

[0163] The variable calculated by the system is the area between theengine speed and an idle speed band around the nominal idle speed. Thisarea is evaluated between the moment when the engine speed crosses thenominal idle speed and the end of the return to idle. It is calculatedpositively, whether the engine speed is higher or lower than the idlespeed, as shown in FIG. 25.

[0164] This area is proportional to the duration of the return to idleas well as to the initial flare value, which is a good reflection of thedriver sensitivity to this aspect of starts: even if the flare iscorrect but lasts for too long, the driver will lower his rating, theworst case being a “high hung up flare”.

[0165] The rating of this start characteristic is represented in FIG.26.

[0166] For each of tip-out and start ratings the values shown in tables3 to 5 are used to obtain an overall rating, the start rating beingdivided between cranking time (table 4) and start quality (table 5).TABLE 3 average max accel total delta decrease oscillations accel TOOL'S(g/s) (g) (g) TOTAL RATING Measurement 0.74 1.46 0.6 Ideal value 1.360.82 0.72 Mark (0 to 1) 0.64 0.23 0.7 Weight 1 1 1 1.47 5.4/10 Driver'scomment: harsh Sportivity: 7661

[0167] TABLE 4 cranking TOOL'S time (s) RATING Measurement 0.585 Idealvalue 0.31 Mark 7.46 7.46/10

[0168] TABLE 5 decrease rise regularity flare flare TOOL'S regularity(basis rating) value area TOTAL RATING Measurement 1.72 149 505 1585Ideal value 1 0 200< × <500 <150 Mark 0.95 −0.49 0.99 0.57 Weight 1.5 10.75 0.75 2.1 5.4/10

[0169] In each case a transfer function as shown in FIG. 12 converts therating to a rating out of 10.

[0170] It will be appreciated that any aspect of driveability can berated in a similar way, broken down into appropriate facets and withempirically observed transfer functions. It will further be appreciatedthat additional or alternative vehicle performance measurements can bemade and further derivations can be made based on those measurements.Calibration/development of engines based on the system can be carriedout in any appropriate manner based on the high level, useful data andobservations automatically available according to the system. The dataobtained can be processed in any appropriate manner and additionalprocessing or identification steps can be carried out on it to derivefurther information from the data obtained.

1. A method of assigning a driveability rating to a predeterminedmaneuver in a vehicle comprising the: identifying a plurality ofmaneuver facets; recording vehicle performance data for the manoeuvre;deriving a local rating for each maneuver facet from the vehicleperformance data; and combining the local ratings to provide an overalldriveability rating for the maneuver, in which the local rating isderived using a predetermined transfer function.
 2. A method as claimedin claim 1 in which the transfer function is derived empirically.
 3. Amethod as claimed in claim 1 in which a vehicle driver driving stylefactor is derived from the recorded vehicle data and the local rating isderived taking the driver driving style factor in account.
 4. A methodas claimed in claim 1 in which the gear ratio for the maneuver isrecorded, a related gear ratio factor identified and the local rating isderived taking the gear ratio factor in to account.
 5. A method asclaimed in claim 1 in which the local ratings are weighted prior totheir combination to form an overall driveability rating.
 6. A method asclaimed in claim 1 in which the combined local ratings are subjected toan overall transfer function to provide the overall driveability ratingfor the maneuver.
 7. A method as claimed in claim 1 in which the localrating is derived from a predefined valid portion of the vehicleperformance data.
 8. A method as claimed in claim 7 in which thepredefined valid portion of the vehicle performance data is processed toprovide representative data prior to derivation of the local rating. 9.A method as claimed in claim 8 in which the processing step includes alinearization step.
 10. A vehicle development method comprising:assigning a driveability rating for a maneuver as set out in claim 1,comparing the assigned rating with a target value and, if the targetvalue is not met, identifying the respective local ratings for eachfacet of the maneuver, identifying those local ratings not meetingrespective target sub-values and identifying related vehicle performancefactors requiring development.
 11. A driveability rating system for avehicle comprising, for a predetermined maneuver having a plurality ofmaneuver facets, vehicle performance data for the maneuver recorded in adata store or processor, a local rating is derived by a data processorfor each facet from the vehicle performance data; wherein the localratings are combined in the processor to provide an overall driveabilityrating for the maneuver in which the local ratings are derived usingrespective predetermined transfer functions.
 12. A method of assigning adriveability rating to a predetermined maneuver in a vehicle comprising:deriving a rating function for a predetermined rating value; identifyingan error or proportional value between the derived function and ameasured function; and deriving a rating based on the error orproportional value.